The invention relates to a valve of the type in which there is arranged, within a valve housing, a valve body with a bore therethrough, and in which the valve body is rotatable about an axis of rotation perpendicular to the bore, and thereby can be rotated into a position in which the bore communicates with an inlet and an outlet of the valve housing. In the open position the inlet, the bore of the valve body and the outlet form the complete bore of the valve along a common valve axis. The valve is closed by rotating the valve body so that the longitudinal axis of the bore is transverse to the valve axis. The valve body then seals against the inlet, outlet or both the inlet and the outlet.
The valve type stated is characterized by the entire cross section of the bore being open when the valve is fully open. This allows, for example, cleaning pigs to be passed through a pipe line in which such valves are incorporated. Valves of this type may also be made with a reduced bore in the valve body.
Different embodiments of the valve type in question distinguish themselves from one another by the shape of the valve body and the sealing between the valve body and the inlet/outlet.
In so-called ball valves a ball-shaped valve body is used, sliding sealingly onto annular sealing surfaces of the valve housing, at the inlet and outlet, respectively. Each sealing surface is in a plane, the sealing plane, forming a right angle with the valve axis, and the two sealing planes are parallel. The contact pressure is determined by spring force, or hydraulically.
Another embodiment of the valve type described initially, is known from NO patent No. 170239, in which each sealing plane forms an oblique angle with the valve axis. At each sealing surface there is arranged, in the valve housing, a valve seat in the sealing plane. On two opposite sides the valve body is provided with annular sealing bodies in planes forming a corresponding, oblique angle with an axis which is perpendicular to the axis of rotation and typically also perpendicular to the bore of the rotational body. A sealing body is brought to bear sealingly on a valve seat, by rotation of the valve body about its axis of rotation. This embodiment has the advantage that the contact force between the sealing body and the valve seat is determined by the torque of the valve body. Another advantage is that a sealing can be achieved at both valve seats, i.e. both at the inlet and at the outlet, thereby ensuring closing of the valve. Moreover, the valve seals equally well in both flow directions.
One quality of this known construction is that positive pressure at the inlet/outlet provides a torque acting on the valve body, and seeks to rotate it towards its open position. The torque/opening torque increases with the inclination of the sealing plane and with the pressure. Therefore, for valves that are to be used by mean to high pressures, it is desirable for the smallest angle of the sealing plane to the valve axis to be as large as possible, i.e. as close to a right angle as possible.
Another quality of this known construction is that a positive pressure inside the valve housing provides a torque at the inlet/outlet, which acts on the valve body, seeking to rotate it towards the closed position. The torque/opening torque increases with the slope of the sealing plane and with the pressure.
If the cooperating sealing surfaces are positioned in the sealing plane, an undesired wedging effect arises between the sealing body and the valve seat. This can be avoided by increasing the slope of the sealing plane, i.e. by reducing the angle between the sealing plane and the valve axis, but, as mentioned, this requires a greater torque on the valve body in order to achieve sealing.
Another and better solution consists in forming the annular cooperating sealing surfaces of the valve seat and the sealing body so that they form a right angle with the sealing plane. In NO 170239 cooperating sealing surfaces are viewed in a plane section perpendicular to the axis of rotation of the valve body, and so that the valve axis is located in the plane of section. In the patent document, cooperating sealing surfaces are prescribed to follow a circle M with a radius R about a centre N at a distance from the axis S of rotation of the valve body. Thereby an angle is obtained between the sealing surfaces and the sealing plane, as mentioned. Transferred to a 3D valve seat and sealing body this means that the cooperating annular sealing surfaces form part of a ball surface (M) with a radius (R) and its centre at N.
It has turned out that a valve made in accordance with NO 170239 does not maintain tightness, except by moderate pressures. By elevated pressure a leak is created, typically where the distance to the axis of rotation of the valve body is the largest.
The object of the invention is to remedy said weaknesses, by a valve of the type known from NO 170239, and make such a valve usable for mean and higher pressures.
The object is realized through features as stated in the following specification and the subsequent claims.
The sealing problem by the valve type known from NO 170239 is assumed to be connected to the fact that the distance from the axis of rotation of the valve body to the cooperating contact surfaces varies because of said inclination of the sealing plane. When the valve body is rotated by a specific torque towards its closed position, a varying torque arm leads to a varying contact force along the contact surfaces, so that there will be the least contact force where the distance to the axis of rotation of the valve body is at its greatest. As the pressure is increased at the inlet/outlet, the valve housing and the valve body are deformed, and leaks occur in areas with least contact force between the cooperating contact surfaces.
According to the invention it is an essential feature that cooperating sealing surfaces have different curvatures. This is in contrast with the valve known from No 170239, in which is prescribed that both sealing surfaces follow the same circle or ball surface. Difference in curvature can be achieved by each sealing surface forming part of a ball surface of a different radius, one sealing surface forming part of a conical surface, while the cooperating sealing surface forms a part of a ball surface, part of a parabolic surface or part of a hyperbolic surface. An effect of this is that the contact between cooperating sealing surfaces is effective along a contact line instead of a contact surface when the valve body is subjected to a torque. If the torque is increased, the surface pressure rises strongly along the contact line. Where the contact force is the greatest, i.e. in the areas where the distance to the axis of rotation of the valve body is the smallest, the material in the valve seat and the sealing body is deformed, so that the contact surface increases. This leads, at the same time, to an greater contact force along other parts of the contact line.
One of the two cooperating contact surfaces may with advantage be made elastically resilient, so that a pre-tensional force is achieved because of the torque applied to the valve body in the closing. A resilient valve seat may, for example, be achieved by the implementation of a comparatively large extension in the longitudinal direction of the valve. In a preferred embodiment an elastically resilient gasket is positioned in a groove in the valve seat. The sealing body then comes to rest against the gasket. An elastically resilient valve seat or an elastically resilient gasket will compensate for the variation in contact pressure along the contact line and for changes in dimensions and shapes of the valve housing and valve body by increased pressure, temperature change, change in torque and external forces.
An inlet and an outlet of circular cross-sections are elliptical with respect to the sealing plane which forms an oblique angle with the valve axis. Therefore, the contact surfaces within the valve seat and the sealing body should, theoretically, constitute part of an elliptic cone surface, part of an elliptic parabola or an elliptic hyperbola. By a relatively small inclination of the sealing plane, i.e. when there is little angular deviation between the axis of symmetry and the bore axis, rotationally symmetric contact surfaces like a ball, cone, parabola or hyperbola, will make a sufficient approximation.
The axis of symmetry of the contact surfaces is perpendicular to the sealing plane and is positioned to the side of the axis of rotation of the valve body, as known in itself from NO 170239.